1. Field of the Invention
The present invention relates to a single-user closed-loop transmit beamforming (SU-CLTB) scheme of a Multiple Input Multiple Output (MIMO) system, and more particularly, to a single-user closed-loop transmit beamforming (SU-CLTB) scheme of Multiple Input Multiple Output (MIMO) system with the transmit beamforming scheme employing a codebook.
2. Description of the Related Art
OFDM (Orthogonal Frequency Division Multiplexing) is a technology of multiplexing data in a frequency domain. Modulation symbols are carried by multiple frequency sub-carriers. The total bandwidth in an OFDM system is divided into narrowband frequency units called subcarriers. The number of subcarriers is equal to the FFT/IFFT size N used in the system. In general, the number of subcarriers used for data is less than N because some subcarriers at the edge of the frequency spectrum are reserved as guard subcarriers. In general, no information is transmitted on guard subcarriers. The advantage of OFDM (Orthogonal Frequency Division Multiplexing) over other transmission schemes is the robustness to multipath fading. The multipath fading in time domain translates into frequency selective fading in frequency domain. With the cyclic prefix or zero prefix added, the inter-symbol-interference between adjacent OFDM symbols are avoided or largely alleviated. Moreover, because each modulation symbol is carried over a narrow bandwith, the modulation symbol experiences a single path fading. Simple equalization scheme may be applied to combat frequency selection fading.
Multiple Input Multiple Output (MIMO) schemes use multiple transmitting antennas and multiple receiving antennas to improve the capacity and reliability of a wireless communication channel. A MIMO system capacity increases a function of K where K is the minimum of number of transmit antennas (M) at transmitter and receive antennas (N) at receiver, i.e. K=min(M,N). The transmitted signals are received at the four receive antennas. Spatial signal processing is performed on the received signals in order to recover the four data streams. An example of spatial signal processing is V-BLAST which uses the successive interference cancellation principle to recover the transmitted data streams. Other variants of MIMO schemes include schemes that perform some kind of space-time coding across the transmit antennas (e.g. D-BLAST) and also beamforming schemes such as SDMA (Spatial Division multiple Access). In addition, MIMO may be implemented with transmit/receive diversity scheme and transmit/receive beamforming scheme in order to improve the link reliability or system capacity in wireless communication systems. The MIMO channel estimation consists of estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. Therefore, the channel for M×N MIMO system consists of an N×M matrix:
  H  =      [                                        a            11                                                a            12                                    …                                      a                          1              ⁢              M                                                                        a            21                                                a            22                                    …                                      a                          2              ⁢              M                                                            ⋮                          ⋮                          …                          ⋮                                                  a                          N              ⁢                                                          ⁢              1                                                            a                          M              ⁢                                                          ⁢              2                                                …                                      a            NM                                ]  where matrix H is the MIMO channel matrix and aij represents the channel gain from transmit antenna j to receive antenna i. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots are transmitted from each of the transmit antennas.
A contemporary transmit beamforming in wireless systems may be done with either closed-loop or open-loop manners. Beamforming is a technique of signal processing performed with transmitters arrays or receivers arrays and may control the transceiving direction and sensitivity of a transceived signal. During transmitting a signal, transmit beamforming may increase a power in the direction along which the signal is to be transmitted. A transmission gain may be achieved by the transmit beamforming process comparing to an omnidirectional transmission.
Open-loop system is typically well suited for TDD (Time Division Duplexing) system. Open-loop system does not require any feedback of channel information. Therefore, less overhead is introduced in open-loop system. The disadvantage of open-loop system however is that an open-loop system needs to constantly conduct phase calibration in order to compensate the phase difference between transmission and reception RF (radio frequency) chains among multiple transmit antennas. Another disadvantage of the open-loop system is that the open-loop system requires a constant uplink phase reference such as an uplink pilot, this requirement may induce an excessive feedback overhead. The process of phase calibration is generally costly, and sensitive to radio channel environment.
Closed-loop, on the other hand, does not require phase calibration process. The closed-loop system however requires the channel feedback to the transmitters. Therefore, overhead is significantly increased in closed-loop system comparing with the open-loop system. Additionally, the closed-loop system may be sensitive to the feedback channel error due to either feedback delay or fast channel variation. Typically, FDD (Frequency Division Duplexing) employs closed-loop transmit beamforming scheme.